Have Frame::tree() return a reference

Source/core/dom/DocumentInit.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 77 matching lines @@
 {
     ASSERT(frameForSecurityContext());
     return frameForSecurityContext()->settings();
 }
 
 Frame* DocumentInit::ownerFrame() const
 {
     if (!m_frame)
         return 0;
 
-    Frame* ownerFrame = m_frame->tree()->parent();
+    Frame* ownerFrame = m_frame->tree().parent();
     if (!ownerFrame)
         ownerFrame = m_frame->loader()->opener();
     return ownerFrame;
 }
 
 DocumentInit& DocumentInit::withRegistrationContext(CustomElementRegistrationContext* registrationContext)
 {
     ASSERT(!m_registrationContext);
     m_registrationContext = registrationContext;
     return *this;
@@ skipping 18 matching lines @@
     return m_contextDocument;
 }
 
 DocumentInit DocumentInit::fromContext(WeakPtr<Document> contextDocument, const KURL& url)
 {
     return DocumentInit(url, 0, contextDocument, 0);
 }
 
 } // namespace WebCore